1. Field of the Invention
This invention relates to antifreeze/coolant additives useful in hard water, and more specifically to antifreeze/coolant additives comprising polycarboxylates.
2. Description of the Prior Art
Antifreeze/coolant technology in North America uses silicate as a corrosion inhibitor. Silicates are particularly useful in protecting aluminum automotive cooling system components. The silicate corrosion inhibitors generally also use a phosphate, usually in the form of an alkali metal salt, to help protect metal cooling system parts and also as a buffer to control the pH of the coolant.
Often phosphate salts are used to help maintain a stable alkaline environment from which multiple corrosion inhibitors can most effectively function. Thus, the degradation of silicate and phosphate in an antifreeze/coolant can negatively impact the overall performance of the antifreeze/coolant.
Traditionally antifreeze/coolant is sold at nearly one-hundred percent glycol content. This concentrated packaging allows for flexibility so that the user can dilute the antifreeze/coolant, as needed, with available water to obtain the required freeze/boil protection. However, corrosion protection is needed over the entire dilution range.
Recently, there has been an increase in concern over the quality of water used to dilute the antifreeze/coolant. Water quality varies greatly with geographic location, population and degree of industrialization.
One of the major problems is hard water control. The U.S. Geological Survey defines hard water in parts per million of ionic calcium. Magnesium is also commonly considered to be a hardness ion. Moderately hard water is from 25 to 50 ppm and hard water is defined as 50 to 75 ppm. Very hard water is considered to be above 75 ppm. At below 25 ppm, the soft water designation is used.
When a typical North American antifreeze/coolant containing silicate and phosphate is mixed with very hard water, copious precipitates develop in a short period of time. Hard water salts can cause maintenance and operations difficulties at automotive manufacturing facilities. These precipitates may clog an automotive cooling system, resulting in reduced coolant flow, increased engine operating temperatures and shorter service life. These precipitates may circulate through the entire cooling system promoting erosion and wear, e.g. water pump damage through increased cavitation and seal abrasion.
The presence of hard water and antifreeze/coolant in an automotive cooling system may also lead to scale formation. The scales can be formed from alkaline earth metal carbonate and phosphate deposition. These inorganic films tend to inhibit thermal transfer and thus reduce the efficiency of the cooling system. Inhibiting scale formation has long been a concern in aqueous cooling systems. For, example, U.S. Pat. No. 3,663,448 discloses scale inhibition for industrial cooling waters using amino phosphonate and polyacrylic acid compounds. U.S. Pat. No. 3,948,792 discloses an aqueous additive mixture to reduce and modify the amount of silicate scale formed in automotive cooling systems.
In addition to the thermal, abrasive and physical problems presented by hard water precipitates, their chemical formation depletes the initial antifreeze/coolant. The hard water precipitates comprise silicate and phosphate salts. By reducing the available quantity of these corrosion inhibitors, the ability of the resultant liquid antifreeze/coolant to provide adequate corrosion protection, particularly for aluminum, is uncertain. The result is dependent upon the hardness of the water used and the initial silicate concentration of the antifreeze/coolant.
In Europe, hard water is more prevalent than in North America. European antifreeze/coolant technology, while commonly using silicate corrosion inhibitors, differs from that of North America in that the technology concentrates on reduction of silicate precipitation. European patent 245557 discloses the use of a variety of compounds including sodium polyacrylate to prevent alkaline earth metal silicate precipitation. However, in this patent phosphate is not used as a buffer in the coolant, thus simplifying the precipitation issue.
U.S. Pat. No. 4,487,712 discloses the use of polyacrylic acid as a silicate stabilizer to inhibit gelation. Gelation is a silicate depletion mechanism which occurs separately from hard water precipitation.
In spite of these disclosures, there remains a need for an antifreeze/coolant additve which prevents hard water precipitates and deposits upon dilution with hard water.